Recent proof-of-concept of electrolytes enabling successful plating and stripping of calcium metal catalyzed research exploring the feasibility of a rechargeable battery technology based on calcium. Nonetheless, the investigation of suitable positive electrode materials has been plagued with a number of hurdles. On one hand, the lack of standard electrolytes and protocols may result in side reactions inducing misinterpretation of the electrochemical response achieved, which makes real bulk activity difficult to elucidate unless complementary characterization techniques are used. On the other, testing of materials similar to those used in the Li-ion battery field which would enable reversible intercalation/de-intercalation of calcium has to date led to the identification of few active compounds with diverse success. Electrochemical extraction of calcium in some ternary transition metal ions is feasible but the reversibility of the process is more difficult to achieve, which is likely related to strong solvation of calcium ions, with reactions sometimes involving solvent co-intercalation and high cell overpotential.
Overall, there is a long and winding road to follow before reliable proof-of-concept can be achieved and technological prospects evaluated. Development of reliable experimental setups, including reference and counter electrodes, coupled to complementary characterization techniques, as well as computational tools, is mandatory if steady progress is to be achieved.
